Gas purification process

ABSTRACT

TREATMENT OF RAW HYDROGEN TO REMOVE LOW BOILING IMPURITIES SUCH AS CARBON MONOXIDE BY SCRUBBING THE RAW HYDROGEN WITH LIQUEFIED PROPANE.

Nov. 2, 1971 F.`KURATA E'rAL 3,616,600

GAS PURIFICATION PROCESS Filed April 4, 1969 INVENTOR. 5950 Kamm BY660,965 Wirf/Fr nited States Patent ce 3,616,600 Patented Nov. 2., 1971Int. Cl. B01d 47/0-6 U.S. Cl. 55-48 5 Claims ABSTRACT OF THE DHSCLOSURETreatment of raw hydrogen to remove low boiling impurities such ascarbon monoxide by scrubbing the raW hydrogen with liquefied propane.

This application is a continuation-impart of our prior and copendingapplication S.N. 606,900, filed Jan. 3, 1967 and now U.S. Patent No.3,455,116.

This invention relates to the purification of low boiling gases.Generally the invention relates to the removal of low boiling impuritiessuch as carbon monoxide from a raw hydrogen stream.

Raw hydrogen consists of about 95 to 98 mol percent hydrogen and about 5mol percent or less of carbon monoxide as the major impurity with tracesof methane and other hydrocarbons. In some instances the raw hydrogenmay contain as much as l or 2 mol percent nitrogen in addition to thecarbon monoxide. The raw hydrogen may be produced as a by-product ofvarious refinery processes or as the primary product of a processdesigned for hydrogen synthesis. Raw hydrogen may be used directly as afeedstock for various processes such as hydrogenation.

Various degrees of purification may be required to provide hydrogensuitable for other purposes. For example, liquefied hydrogen is commonlyused to fuel rocket engines. At the low temperature of the liquidhydrogen production, carbon monoxide may solidify and precipitate tofoul the processing equipment system. To avoid fouling, hydrogen purityof greater than 99.995 percent is desirable for hydrogen liquefaction.

A cryogenic process for the purification of hydrogen is brieflydescribed in Chemical Engineering, May 13, 1963, pages 150 to 152. Thispurification process involves two different liquid solvents to removethe impurities from the entering hydrogen feed stream. The first solventused is liquid methane which removes nitrogen and carbon monoxideimpurities. The second solvent is liquid propane which is then used toremove methane from the purified hydrogen stream, not hydrogen or carbonmonoxide which pass overhead with the purified hydrogen from the propanescrubbing step. The present invention utilizes a single liquid solventsuch as propane or a higher boiling hydrocarbon or mixtures thereof toremove initially the nitrogen or carbon monoxide from the crude gasstream. Thereafter, no solvent treatment is necessary with a differentsolvent. Since a single solvent is utilized there is no necessity fortreating the stream with a second solvent, thus materially improving theeconomy of the process. Furthermore, the use of propane permits a muchwider temperature range in the absorber.

An object of the present invention is to provide a process for theremoval of nitrogen and/ or carbon monoxide from raw hydrogen.

Another object is to provide a process for making highly pure hydrogen.

Other objects and advantages of the present invention will becomeapparent from the accompanying description and disclosure.

In the process of the present invention, the major low boilingimpurities, nitrogen or carbon monoxide, are removed from a raw hydrogenstream containing same by scrubbing with a suitable liquefiedhydrocarbon solvent such as propane. The solvent should be such thatreplacement of impurity by solvent in the scrubbed pure gas gives amixture more amenable to final cleanup, e.g., propane can be much morereadily removed from hydrogen than can carbon monoxide .(impurity) fromhydrogen (product). Solvents of the paraflinic, naphthenic, and aromatichydrocarbon families which are free from ethylenic unsaturation andmixtures thereof that remain liquid at a temperature down to about -50or to about -100 C. are suitable. Generally such solvents are relativelylow boiling, usually having a boiling point below 60 C. at atmosphericpressure.

The product from the scrubber or absorber will then comprise purifiedhydrogen and a small amount of propane or other solvent, usually above 5mol percent. The residual propane or other solvent in the purifiedhydrogen is then removed by cooling and phase separation or by otherconventional techniques such as adsorption on activated carbon or otheradsorbent bed whereby purified hydrogen is produced having a purity of99.995 percent or higher.

Since the capacity and affinity of the adsorbent bed is much greater forhydrocarbons than for carbon monoxide or nitrogen, the adsorption, ifthis technique is used, can be carried out at a higher temperature andremoval of propane is much more complete than for nitrogen or carbonmonoxide. The purified hydrogen can also be permeated through quartztubes or other material to remove the solvent and produce the puriedproduct.

Another important aspect of this invention takes advailtage of thereverse solubility of hydrogen in a hydrocarbon solvent. For most gases,the solubility of the gas in a hydrocarbon solvent increases withdecreasing temperature at a given pressure. In the case of hydrogen, itssolubility decreases with decreasing temperature over the majority oftemperature range of interest for processing purposes.

In the initial scrubbing or absorbing process, some hydrogen will becomedissolved in the solvent in addition to the carbon monoxide or nitrogen.Unless some steps are taken, the hydrogen dissolved in the solvent willbe lost when the carbon monoxide or nitrogen is separated from the.solvent. The dissolved hydrogen can be recovered by flashing thehydrocarbon solvent eflluent from the scrubber or absorber to lowerpressure in two or more stages at different temperatures. The gasesremoved by the low temperature flash will contain most of the dissolvedhydrogen because of its low solubility. This gas can be recycled to thehydrogen feed stream. The gases flashed off at higher temperatures willbe mostly carbon monoxide or nitrogen because of its lower solubility athigher temperatures. The temperatures and pressures selected for theseflashes can be optimized to give maximum recovery of the hydrogen(product) from the hydrocarbon while minimizing vaporization of carbonmonoxide and nitrogen.

The phase equilibrium relationships for the complete system must beknown to take full advantage of the present technique, e.g., in thehydrogen-carbon monoxide-propane system the hydrogen demonstrates thereverse solubility phenomenon which favors good scrubbing action atreasonably high temperatures when conducted at elevated pressures. Also,inthe same system, for the type of operation depicted in theaccompanying process flow diagram of FIG. 1 of the drawings, one musttake care to operate above the locus of critical solution points whichcorresponds closely to the vapor pressure of carbon monoxide.Accordingly, FIGS. 2 and 3 of the drawings show the ternary compositiondiagrams at selected temperatures and pressures for the hydrogen-carbonmonoxide-propane system. These diagrams enables one to set theconditions for operation of the process as described in connection withthe process illutrated in FIG. 1.

The accompanying FIG. 1 is a diagrammatic owsheet illustrating aspecific embodiment of the present invention. The accompanying data ofTables I and II below provide a material balance and operatingconditions as a typical example of design operation for the process ofFIG. l. According to the drawing, raw hydrogen entering through conduit11 is purified to produce a hydrogen product of 99.995 percent purity.The raw hydrogen enters the process at 510 p.s.i.a. and is cooled inprecooler 9. The cooled gas is passed into absorber A. An essentiallypure liquid propane scrubbing stream containing not more than 0.0003percent carbon monoxide is passed through a cooler and conduit 12 intoabsorber A. Absorber A comprises a conventional absorption tower with asuitable number of trays or height of packing. A gaseous hydrogen streamis removed via conduit 13 from the top of absorber A saturated withpropane but highly purified with respect to carbon monoxide. Thishydrogen stream is cooled to 50 C. in cooler 14 and passed into phaseseparator C through conduit 15. A hydrogen product of greater than99.995 percent purity on a propane free basis is Withdrawn as a gas fromthe top of phase separator C through conduit 16.. Although not shown inFIG. 1, the product gas in conduit 16 could be further purified bypassing through a bed of activated carbon or other suitable adsorbent toincrease purity.

4 energy by means not shown to boil the propane. A pure propane streamis withdrawn from the bottom of stripper B through conduit 22 andrecycled through the stripper feed-effluent heat exchanger to the top ofthe absorber A.

A hydrogen and carbon monoxide gas mixture saturated with propane iswithdrawn from the top of stripper B through conduit 23. The majority ofpropane is condensed from the hydrogen and carbon monoxide gas bycooling in heat exchanger 24. In the process of FIG. 1, the operatingtemperature and pressure of stripper B are sufficiently high that alarge fraction of the cooling can be provided by cooling tower waterwhich conserves on the process requirement for relatively expensiverefrigeration.

The cooled stripper overhead stream is passed through conduit 26 tophase separator D. The condensed propane is recycled from the bottom ofseparator D through conduit 29 to the stripper B via conduits 18 and 21.A gaseous mixture of hydrogen, carbon monoxide, and propane is withdrawnfrom the top of separator D through conduit 27 and passed out of theprocess for use as fuel or other processing requirements.

If nitrogen is present in the raw hydrogen stream, it will be removedalong with the carbon monoxide in conduit 27 of FIG. l.

The process owscheme of FIG. l was based upon the assumption ofrelatively low value of ray hydrogen and propane feedstreams andrelatively high value for the fuel stream 27, In some processingsituations, the differential between the value of the ray hydrogen andpropane make-up streams and the value of the fuel stream 27 may beadequate to justify the expense of additional processing steps toreclaim most of the hy- HYDROGEN PURIFICATION PROCESS TABLE L MATERIALBALANCE Stream No 11 12 13 15 16 17 18 19 21 22 23 26 27 29 Mass flow,(lb. mols unit time... 100 1586. 94 65.1 65.1 52. 04 13.06 1G34. 9 12.01970.0 1586. 04 383 383 59. 9 323.1 Temp., C 0 0 0 50 8 -8 0 25 75 82 750 0 0 Comp., mol percent: l

Hydrogen 95. 0 Nil 80. 596 80. 590 90. 095 2, 0850 2.60 Nil 2, 53 Nil13. 02 13.02 71. 7 2. 13 Carbon mOnOXlde.. 5. 0 Nil 0. 004 0. 004 0.00472 0. 00039 0.305 Nil 0. 389 Nil 2. 00 2. 00 8. 3 0. 82 Propane 0. 099. 999+ 19- 40 19. 40 0. 00071 97. 9146 97. 095 99+ 97. 08 99+ 84. 9884. 98 20. 0 97. 05

TABLE IL MAJOR PROCESS VESSEL OPERATING CONDITIONS Vessel Number A B C DE F G Vessel Name Low High Refi-ig- Y st age stage eiant Vapor Yaporsuction suction propane liquid liquid knockout knockout surge AbsorberStripper separator separator drum drum tank Operating temp., C;

Top 0 75 Bottom o s2 -50 0 56 -5 5o Operating pressure, p.s.l.a 510 480500 470 7 30 260 A liquid stream predominantly comprised of propane isrecycled from the bottom of separator C through conduit 17 to absorber Ato a separation stage well below the top of the asborber. Heatexchangers (not shown) are utilized to transfer energy from the feedstreams to the efuent streams of separator C to economize on processenergy requirements.

Liquid propane is removed from the bottom of absorber A through conduit18 and passed through the propane stripper feed-eiuent heat exchanger(not shown), and then is mixed with process make-up propane from conduit19 and recycle propane from conduit 29 and is passed into the top of thepropane stripper B through conduit 21. The propane stripper B comprisesa conventional tower with suicient mass transfer separation stages toprovide a pure propane bottoms product which contains no more than0.0003 percent carbon monoxide. The carbon monoxide is stripped from thepropane by propane vapor which is generated at the bottom of thestripper B by addition of drogen and/or propane contained in Stream 27.Such additional processing steps comprise cooling the flow throughconduit 27 to between -10 and 50 C. to condense the majority of propane.The cooled stream would then be passed into an additional phaseseparator. Because of the reverse solubility of the hydrogen, a vaporphase comprising predominantly hydrogen with carbon monoxidecontamination could be recycled from the top of this additional phaseseparator to the absorber A. Because of its higher solubility, adisproportionately higher concentration of carbon monoxide would bewithdrawn with the liquid propane stream from the bottom of thisadditional phase separator.

The liquid propane with carbon monoxide contamina tion from the bottomof the phase separator would be flashed to provide a vapor phasecontaining essentially all of the contaminant carbon monoxide removed inthe process. The vapor phase would be rejected from the process and theliquid propane phase would be recycled to the top of the stripper B.

The above is a description of one specific embodiment of the inventionbut it is to be understood that other operating conditions can beutilized to achieve similar results. Insofar as the temperature andpressure of the process of this invention are concerned, two liquidphases form in the propane-carbon monoxide system below -138.7 C. Theupper temperature limit will be near the critical temperature ofpropane, 100 C. The formation of the second liquid phase below -138.7 C.can be avoided by proper selection of pressure or propane circulationrate. Therefore, absorber A can be operated down to the temperaturewhere solid forms. In order to avoid the formation of a second liquidphase, absorber A can be operated at a pressure lower than the threephase locus pressure at temperatures below -138.7 C. Or, the propanecirculation rate can be set so the carbon monoxide concentration will beless than the equilibrium concentration in the propane rich liquid layerwhen two liquid phases are present.

This means that above -138.7 C. the absorber A operating temperature andpressure can be set independently. Below -138.7 C. the tower pressuremust be below the three phase pressure or the propane (or solvent)circulation rate can be set so that the second liquid phase does notform.

By operating at low temperature and high pressure in phase separator Cthe vapor pressure of propane is so low that the hydrogen may have thedesired purity without further purification as is shown in theflowscheme of FIG. 1 and the material balance and operating conditionsof Tables I and II.

Economics will be the controlling factor in establishing the optimumoperating temperatures and pressures for the various processing steps.For example, the required circulation rate for propane to the absorbercan be decreased approximately 50 percent by increasing the absorberoperating pressure from 500 p.s.i.a. to 1000 p.s.i.a. However, thisdecreased expense for propane circulation must be balanced againsthigher initial cost for equipment and higher cost for compressing thefeed gas to the process operating pressure.

Having described our invention, we claim:

1. A process for making pure hydrogen which comprises introducing into ascrubbing zone raw gaseous hydrogen containing carbon monoxide as themajor impurity, introducing into said scrubbing zone liqueed solventcomprising hydrocarbon substantially free from ethylenic unsaturationand having at least 2 carbon atoms per molecule and which boils below 60C. and remains liquid at a temperature of 100 C., maintaining saidscrubbing zone at a temperature, pressure and contact ratio betwen rawhydrogen and liqueiied solvent such that the solvent remains liquid as asingle phase and the raw hydrogen and its carbon monoxide impurityremain gaseous, removing liquelied solvent containing carbon monoxideimpurity therein from said scrubbing zone, and removing from saidscrubbing zone purified gaseous hydrogen.

2. A process for making pure hydrogen which comprises introducing into ascrubbing zone raw gaseous hydrogen containing carbon monoxide as themajor impurity, introducing into said scrubbing zone a liquefied propanesolvent, maintaining said scrubbing zone at a temperature above that atwhich solid forms and below +l C. and under a pressure and a contactratio between raw hydrogen and liquefied propane such that a gas phaseand a single liquid phase are produced, removing liquelied propane fromsaid scrubbing zone containing carbon monoxide impurity from said rawhydrogen, and removing from said scrubbing zone purilied hydrogen.

3. A process for making pure hydrogen which comprises introducing into ascrubbing zone raw gaseous hydrogen containing carbon monoxide as animpurity, introducing into said scrubbing zone liqueled propane,maintaining said scrubbing zone at a temperature between about -l39 C.and about +100" C. and under a pressure and a contact ratio between rawhydrogen and liqueed propane such that a gas phase and a single liquidphase are produced, removing liquefied propane from said scrubbing zonecontaining carbon monoxide impurity from said raw hydrogen, strippingcarbon monoxide from said liqueed propane from said scrubbing zone,recycling stripped propane to said scrubbing zone, removing from saidscrubbing zone purilied hydrogen substantially free of carbon monoxideand containing propane, separating propane from said puried hydrogen andrecovering purified hydrogen as a product of the process.

4. The process of claim 3 in which propane is separated from puriiiedhydrogen by phase separation at reduced temperature.

5. The process of claim 3 in which all of the stripping of carbonmonoxide from liquefied propane is carried out at superatmosphericpressure.

References Cited UNITED STATES PATENTS 3,121,624 2/1964 Matsch et al.55-48 3,239,458 3/1966 Baumann et al. 55-48 REUBEN FRIEDMAN, PrimaryExaminer C. N. HART, Assistant Examiner U.S. Cl. X.R. 55-68; 62-17

